Polymer based compositions are becoming increasingly used in a variety of different technological applications to include vehicle and aerospace applications. Polymer based composites are used in not only structural applications, but also in construction of system components in order to control the electrical conductivity and resistivity of the components. Polymers widely vary in their ability to dissipate static charges and to prevent the composite material from acting as a mere conductor of electrical current.
Polymeric composites with both conductive and non-conductive properties can be chosen by incremental addition of selected materials such as carbon black, carbon fiber, metallic fibers and powders, as well as selecting inherently conductive polymeric compositions such as polyaniline. Further, it is known that by varying the amount of conductive or resistive particles added to the composition, some incremental control can be obtained for establishing the conductive or resistive properties of the material.
Fuel lines, particularly those used in aircraft, should preferably have the ability to accommodate a wide range of electrical performance requirements depending upon the location of the fuel line, the type of fuel line and other factors. It is desirable to have fuel lines that are resistant enough to prevent them from acting as conductors of electrical current such as caused by a lightening strike, yet the fuel lines should be conductive enough to prevent static charge build up that could result in an electrical arc creating a spark within the fuel line.
Various efforts have been made to isolate fuel lines to ensure the fuel lines maintain the desired electrical properties; however, traditional fuel lines are unable to accommodate wide electrical performance requirements when comparing an event such as a lightning strike versus slow but incremental static buildup.
One example of a reference that discloses fuel lines with controlled resistivity includes the International Publication Number WO2009/087372. This reference describes a glass reinforced composite pipe having an electrically nonconductive polymeric resin matrix and an electrically nonconductive reinforcement component such as glass fiber. The composition further includes a selected dispersion of an electrically conductive particulate filler, such as carbon black in which the resistivity of the outer portion of the composite pipe is preferably set between 50 K-ohms per meter length and 4 M-ohms per meter length.
Another reference that addresses electrical conductivity problems associated with fuel carrying pipes or tubes is the European Patent Application Publication No. 0028310. This reference discloses a filament reinforced plastic article having a fluid impermeable wall with a relatively low surface resistivity to prevent build up of electrostatic charge. The article includes overlapping bands of filament coated with a plastic material. A minor portion of the filament in some of the bands is electrically conductive by use of electrically conductive filaments that may be distributed throughout the wall of the article.
Another example of a fuel pipe or tube addressing desired electrical properties is the U.S. Patent Application Publication No. U.S. 2010/011,694. This reference describes ducting comprising at least one multi-layer pipe having respective inner and outer layers made of a composite plastic material, and reinforced by an electrically conductive reinforcement layer. An intermediate layer is provided for stiffening of the pipe when flexed and/or to insulate the pipe. Each of the inner and outer layers has at least one winding with contiguous turns of a material made of a composite thermoplastic. The pipe is able to discharge electrostatic build up through its inner layer, and electrical charges due to lightening are deflected away from the pipe through its outer layer.
While these references may be adequate for their intended purposes, there is still a need to provide fluid conveying tubes or pipes that are economical to manufacture, and may accommodate wide ranges of electrical specifications both for static discharge and lightening protection. Further, there is need to provide a tube or pipe construction that can be shaped and sized for many different applications. For example, there is a need to provide the capability of manufacturing fuel lines for aircraft in which the tubes can be easily sized and shaped to fit within constrained spaces.
One particular problem associated with composite tubing is delamination that can occur between joined sections of tubes or layers among tubes. The most common manufacturing practice is to simply overlap abutting edges of tube sections to be joined. Although initially these overlapping edges may provide an adequate structural connection, vibration and chemical degradation by contact of the fuel with the composite material eventually results in some degree of delamination. Particularly in pipe runs that must follow a torturous path, there may be multiple sections of tubes/pipes that are joined. In these cases, it may be impractical or impossible to use mechanical couplers at each junction due to weight and size constraints. Therefore, there is also a need to provide a composite tube construction in which long continuous pipe runs having complex shapes are created without a concern for delamination between abutting tube sections.
Another limitation associated with composite tubes that may incorporate selected conductive or non-conductive properties is that changing the conductive properties of differing sets of tubes within a fluid delivery system requires reformulation for each set of tubes, since the practice is to add or remove conductive or resistive particles to the compositions. While some incremental control may be achieved to control fluid delivery system conductivity as a whole, it becomes prohibitively complex and expensive to change formulations for many different sections of tubing and fittings within a particular fluid delivery system. Accordingly, incremental control across a single section of tubing, or within different sections of tubing within a system becomes impractical. Therefore, there is also a need to provide an ability to electrically tune distinct tubing and fittings components within a fluid delivery system without having to change formulations.
Although there have been advances made with respect to the chemical aspects of composite materials, most present day aircraft are still constructed with metallic tubing (such as aluminum, stainless steel, or titanium). In order to electrically tune these metallic tubes, various static discharge and electrical isolators are required, which add cost and weight to the aircraft.